This invention relates generally to the field of sight glasses, used to determine the level of fluid in a container without unsealing the container. Typical installations are upon steam boilers or hot water heaters to determine the level of water in the boiler. In such installations the water being measured is under considerable pressure, and the possibility of rupture of the sight glass and subsequent injury to a viewer is always present. It is known to provide a concentric shield having a relatively small slotted opening therein for viewing the tube. In the preferred type of shield, a transparent sleeve is provided which is sealed at the end portions thereof to provide an enclosed space between the sight glass and the shield.
In recent years, sight glasses have been employed in systems conducting corrosive or otherwise chemically active fluids as well as water. While the sight glass itself is relatively chemical inert, the remaining components are usually not, so that with continued use, deterioration occurs with the accompanying danger of leakage of such chemically active fluid.